Cement manufacturing is a critically important industry in the United States and throughout the world. In 2006, U.S. cement plants produced 99.8 million metric tons of Portland cement. Worldwide production accounted for about 2.5 billion metric tons. In the Portland cement manufacturing process, raw materials, which are generally comprised of limestone, sand, clay and iron ore, are fed proportionally into a grinding mill where they are crushed and ground to the desired fineness, and then fed into a rotary kiln system for calcining. The resulting calcined material, called “clinker”, is allowed to cool and then passed into a grinding mill where it is interground with gypsum to produce the cement product.
A significant byproduct of the Portland cement manufacturing process, amounting to up to about 15 percent of the raw materials processed, is cement dust, which includes both the cement kiln dust (CKD) retained from cement kiln exhaust gas by air pollution control devices, and the cement bypass dust produced during the initial crushing and grinding of the raw materials. Cement dust is typically composed of varying concentrations of particles of raw materials, partly to wholly calcined raw materials, reaction process intermediates, fine clinker and inorganic fuel solids. Its high lime content and extremely fine particle size combine to render cement dust an extremely hazardous pollutant that has posed an ongoing serious solid waste management problem for the cement manufacturing industry.
The amount of cement dust that can suitably be recycled back into the cement manufacturing process as a feed without detrimentally affecting the efficiency of the process, is generally limited to not more than about 20 percent of the total amount generated. As for the unrecycled portion, only relatively minor amounts of cement dust have thus far been found useful for beneficial applications, such as soil consolidation, waste stabilization/solidification, cement additives, mine reclamation, agricultural soil amendment, liners and covers for sanitary landfills, wastewater neutralization and stabilization, pavement manufacturing, and concrete products. This leaves the major bulk of generated cement dust requiring disposal, typically in land-based disposal sites, such as landfills, waste piles or surface impoundments, which not only is costly and wasteful, but also renders these land sites completely useless for any other application such as agricultural or urban development.
Different approaches have been proposed by the prior art for addressing the cement dust problem. One such approach is to subject the cement dust to various treatment methods designed to render it more suitable for recycling back into the cement manufacturing process. The Sell et al. U.S. Pat. No. 4,161,441, the Cohen et al. U.S. Pat. No. 5,782,973, and the Gebhardt U.S. Pat. No. 6,331,207, are all illustrative of this approach. Sell et al. employ a high-pressure method for compacting the cement dust into mineral oil encapsulated briquettes. The Cohen et al. treatment method involves forming a mixture of the cement dust and a carbon-bearing material into nodules that are then thermally processed in a fluidized bed to reduce the alkali and sulfur content of the cement dust. Gebhardt treats the cement dust with carbon dioxide gas in an aqueous slurry, thereby allowing dissolved alkali and sulfate compounds to be removed from the cement dust solids. All of these treatment methods tend to be costly and complex solutions to the problem with significant energy expenditures.
Another prior art approach to the problem has been to attempt to incorporate the unrecycled cement dust as a component in various cement and concrete products. Cement dust utilization in preparing settable cement compositions is exemplified by the Wills, Jr. U.S. Pat. No. 4,268,316, and the Roddy et al. U.S. Pat. Nos. 7,353,870 and 7,445,669; and in forming construction blocks and other durable masses by the Wills, Jr. U.S. Pat. No. 4,407,677, the Kneller et al. U.S. Pat. No. 4,432,800, and the Riddle U.S. Pat. Nos. 5,366,548 and 5,425,807. In this latter regard, it should be noted that the construction blocks described by Wills, Jr. and the durable mass described by Kneller et al., both consist almost entirely of material other than cement dust and allow for the incorporation of the cement dust component in only very limited amounts, i.e., less than 7 weight percent in the case of Wills, Jr., and not more than 16 weight percent in the case of Kneller et al.
On the other hand, the cement dust-containing construction blocks disclosed by the two Riddle patents are formed from a dry composition containing from 100 to 15 weight percent cement dust with the remainder of the composition being an additive which may be cement, lime, gypsum plaster, polymers, resins, pumice, volcanic ash, sand clay or sand aggregate. The dry composition is blended with water, and the blended material is compressed in a compression zone at a pressure of 1000 to 2500 psi to form the construction block. Thus, while Riddle does teach the possibility of forming construction blocks incorporating a high percentage of cement dust and a low percentage of additive material, this teaching carries with it the proviso that high-pressure forming techniques must necessarily be employed in doing so. There is no hint whatsoever in either of the two Riddle patents of if or how high-percentage cement dust-containing construction blocks might otherwise be produced.